1. Field of the Invention
The invention relates to automatic parametric control facilities and, in particular, to an automatic temperature control therefor.
2. Description of the Prior Art
Known in the art is an automatic temperature control method comprising the steps of furnishing a deviation signal proportional to an algebraic difference between predetermined and actual temperatures of a controlled entity and a control signal functionally dependent on the deviation signal and affecting an actuator in the direction of elimination of the existing deviation of the actual temperature from the predetermined temperature, another step being a continuity check of a controlled entity actual temperature transducer circuit.
A known automatic temperature control device accomplishing the prior art method comprises such series-connected components as a controlled entity temperature setter, a comparison unit whose second input is connected to the controlled entity actual temperature transducer, a deviation signal amplifier and a control unit furnishing a signal functionally dependent on a deviation signal, the output of said unit being electrically coupled to the input of an additional unit. The foregoing device also includes a transducer circuit continuity check unit connected to the output of the transducer (cf. A. I. Telis and G. A. Makarov "Temperature Control of Heating Zones of Plastic Processors," 1976 NIIMASH publication, Series C-4, Foundry Machinery Industry, Moscow, pp. 36-38, in Russian).
The known device is designed to automatically control the temperature of heating zones of physical cylinders of plastic pressure-casting machines.
An output element of the actuator is an electric heater arranged on the outside of the cylinder, while the transducer is a thermocouple installed in the body of the physical cylinder of the casting machine.
The electric heater is designed to heat and melt the processed material, the essential requirement being that the temperature within heating zones must be maintained to a high accuracy to obtain quality items.
Such a requirement is necessitated by the fact that a temperature rise over the predetermined value impairs mechanical characteristics of plastics, increases viscosity of the processed material, and a required casting pressure. Internal stresses may, therefore, arise in fabricated parts. A temperature rise over the predetermined value also results in overheating and decomposition of plastics. Furthermore, a higher casting temperature increases a finished item cooling time and reduces operating efficiency of the machine.
It is of particular importance to maintain the temperature which specified limits in processing polyamides and quite a number of special materials characterized by a narrow temperature range in solid-liquid transition. In such instances thermal control must be accurate to +2.degree..
In the known device an open transducer circuit results in emergency conditions which arise due to the fact that the control unit furnishes at its output a control signal which may take on a random value (from a maximum to a zero level), a disadvantage leading to impermissible heating or cooling of plastics within a physical cylinder of a pressure-casting machine during a production process.
In processing thermosetting plastics the cooling due to an open transducer circuit may cause irreversible hardening of the processed plastic within the cylinder, a limitation increasing downtime and making trouble-shooting a time-consuming operation.
In the known device the transducer circuit continuity check unit merely provides indication of an open circuit to attract the operator's attention.
To exclude emergency conditions while repairing the break, it is necessary to turn on stand-by control devices, which is commercially disadvantageous, or to stop the production process, thus decreasing efficiency in fabrication of plastics.
Furthermore, the transducer circuit continuity check units utilized in the known device are characterized by low noise immunity and introduce additional interference in input circuits of the automatic control device, a limitation substantially reducing reliability of a control process.